A barometric-based approach for estimating an altitude of a mobile device requires (i) an accurate estimate of pressure at a location of the mobile device, (ii) an accurate estimate of pressure at a location of a remote pressure sensor, (iii) an accurate estimate of temperature at a location of a remote temperature sensor, and (iv) a known altitude of the remote pressure sensor, where the location of the remote pressure sensor and the location of the remote temperature sensor may be the same location or different locations. An estimate of an altitude of the mobile device (hmoblie) can be computed as follows:
                                          h            mobile                    =                                    h              sensor                        ∓                                                            RT                  remote                                gM                            ⁢                              ln                ⁡                                  (                                                            P                      sensor                                                              P                      mobile                                                        )                                                                    ,                            (                  Equation          ⁢                                          ⁢          1                )            where Pmobile is the estimate of pressure at the location of the mobile device, Psensor is the accurate estimate of pressure at the location of the remote pressure sensor, Tremote is the accurate estimate of temperature at the location of the remote temperature sensor, hsensor is the known altitude of the remote pressure sensor, g corresponds to the acceleration due to gravity, R is the universal gas constant, and M is the molar mass of air (e.g., dry air or other).
In some cases, the estimate of pressure at the location of the remote pressure sensor is converted to a reference pressure that specifies an estimate of pressure at a reference altitude as follows:
                                          P            ref                    =                                    P              sensor                        ⁢                          exp              ⁡                              (                                  -                                                            gM                      ⁡                                              (                                                                              h                            ref                                                    -                                                      h                            sensor                                                                          )                                                                                    RT                      remote                                                                      )                                                    ,                            (                  Equation          ⁢                                          ⁢          2                )            where Psensor is the accurate estimate of pressure at the location of the remote pressure sensor, Pref is the reference pressure and href is the reference altitude. The altitude of the mobile device, hmobile is computed using Equation 1, where href is substituted for hsensor and Pref is substituted for Psensor The reference altitude, href may be any altitude, and is often set at sea-level.
When two or more reference pressures are available, the reference pressures are combined into a single reference pressure value (e.g., using an average, weighted average, or other suitable combination of the reference pressures), and the single reference pressure value is used for the reference pressure, Pref.
As one of ordinary skill in the art will appreciate from Equation 1 and Equation 2, the estimate of the mobile device's altitude, hmobile, will be accurate so long as the following information is available and accurate: (i) the estimate of the local pressure, Pmobile, (ii) the reference pressure, Pref, and (iii) the estimate of the remote temperature, Tremote. The estimate of local pressure, Pmobile, can be computed by calibrating measurements of pressure by a local pressure sensor of the mobile device as is known in the art. Unfortunately, accuracy of the reference pressure, Pref, and the estimated remote temperature, Tremote, depends on the accuracy of remote sensors that are available.
In some situations, a network of calibrated reference sensors is used, which produces reliable reference pressures and estimates of remote temperature that require no further adjustments. In other situations, less reliable reference sensors are used, which requires determining if the reference pressures and/or estimated remote temperatures are accurate before they are used to compute an estimate of an altitude of a mobile device. Described below are systems and methods for determining if a reference pressure is accurate, and for determining new reference pressures for use in computing an estimate of a mobile device's altitude.